swim: interpret no-payload as nil

Empty string as a no-payload-flag was not a good idea, because
then a user can't write something like:

    if not member:payload() then
        ...

Follow up #3234

src/lua/swim.lua

@@ -338,7 +338,11 @@ end
 --
 local function swim_member_payload_str(m)
     local ptr = swim_check_member(m, 'member:payload_str()')
-    return ffi.string(swim_member_payload_raw(ptr))
+    local cdata, size = swim_member_payload_raw(ptr)
+    if size > 0 then
+        return ffi.string(swim_member_payload_raw(ptr))
+    end
+    return nil
 end
 
 --
@@ -367,7 +371,7 @@ local function swim_member_payload(m)
     local cdata, size = swim_member_payload_raw(ptr)
     local ok, result
     if size == 0 then
-        result = ''
+        result = nil
     else
         ok, result = pcall(msgpack.decode, cdata, size)
         if not ok then

test/swim/swim.result

@@ -429,11 +429,11 @@ s:payload_cdata()
 ...
 s:payload_str()
 ---
-- 
+- null
 ...
 s:payload()
 ---
-- 
+- null
 ...
 s:is_dropped()
 ---
@@ -536,7 +536,7 @@ s:set_payload()
 ...
 self:payload()
 ---
-- 
+- null
 ...
 s:set_payload({a = 100})
 ---
@@ -603,7 +603,7 @@ s:set_payload()
 ...
 self:payload()
 ---
-- 
+- null
 ...
 s:set_payload(100)
 ---
@@ -619,7 +619,7 @@ s:set_payload_raw()
 ...
 self:payload()
 ---
-- 
+- null
 ...
 -- Raw payload setting can be used when MessagePack is not needed,
 -- or already encoded.
@@ -724,7 +724,7 @@ s1_view = s2:member_by_uuid(uuid(1))
 ...
 s1_view:payload()
 ---
-- 
+- null
 ...
 s1_view:incarnation()
 ---
@@ -878,7 +878,7 @@ s1_self:payload() == p
 ---
 - true
 ...
-while s1_view:payload() == '' do fiber.sleep(0.01) end
+while not s1_view:payload() do fiber.sleep(0.01) end
 ---
 ...
 p = s1_view:payload()

test/swim/swim.test.lua

@@ -284,7 +284,7 @@ s1:set_payload({a = 100})
 p = s1_self:payload()
 s1_self:payload() == p
 
-while s1_view:payload() == '' do fiber.sleep(0.01) end
+while not s1_view:payload() do fiber.sleep(0.01) end
 p = s1_view:payload()
 s1_view:payload() == p